Tubes Handbook 18th Edition (EN) - May 2018

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2.2. Chemical Basis – Reaction Mechanisms
Since chlorinated hydrocarbons are not indicated by direct colorimetric reaction, an
oxidative cleavage of the molecule is necessary as a first step. This reaction is done with
potassium permanganate, which forms elementary chlorine. The chlorine then reacts with
the reagent preparation in the indicating layer to produce the colorimetric reaction product.
The measurement of carbon dioxide is done by oxidation of hydrazine hydrate in the
presence of crystal violet as an oxidation-reduction (redox) reaction:
CO 2 + N 2 H 4 ä NH 2 –NH–COOH
Typically carbon dioxide will be present at a substantially higher concentration than any
potentially cross sensitive substances, therefore this reaction is very selective. Possible interferences
by hydrogen sulphide and sulfur dioxide are not expected since these
interferences can only occur with unusually high concentrations.
Another large group of Dräger-Tube reactions is based on pH indicators, for example:
NH 3 + bromophenol blue ä blue reaction product
This type of reaction is valid for basic as well as acid gases.
Compounds containing the C N-group are measured using multiple stage reactions. In the
case of acrylonitrile, the first step is a oxidation. In the next step the cyanide ion reacts with
mercury chloride to form hydrochloric acid and undissociated mercury cyanide. The
hydrochloric acid is indicated in the last partial step of this complex reaction system by means
of a pH indicator. Suitable prelayers are used to ensure a selective measurement. A similar
reaction principle is also used in the most sensitive hydrogen phosphide (i. e. phosphine)
tubes, Phosphine 0.01/a. The hydrogen phosphide also reacts with mercury chloride, but
in this case yields mercury phosphide and hydrochloric acid. Again, the hydrochloric acid is
indicated by means of pH-indicator.
Most hydrides of the elements from group III or V of the periodic table (e.g. borane or
arsine), react because of their reducing characteristics with gold salts by forming
elemental gold.